The present invention pertains to gas turbine engines and, more particularly, to a method of operating same to reduce carbon monoxide and unburned hydrocarbon emissions.
The present era of environmental awareness has spurred governmental regulations limiting the permissible exhaust emissions from gas turbine engines. Some of the more severe requirements relate to carbon monoxide (CO) and unburned hydrocarbon (HC) emissions. These emissions have traditionally been the greatest at ground idle conditions where the combustor inlet temperature and pressure, and the combustor fuel-to-air ratio, are relatively low.
As gas turbine powered aircraft are designed for operation from shorter runways, the emissions problem will become more acute. The reason is that short-field aircraft must be overpowered (i.e., higher installed thrust-to-aircraft weight ratio) compared to the more conventional take-off and landing aircraft. For example, during taxi operation the engine power setting must be reduced abnormally to avoid overloading the aircraft brakes, particularly on icy runways. As the engine throttle is pulled back to this abnormal position, the combustor inlet temperature drops (due to lower work input of the compressor) resulting in inefficient burning and increased exhaust emission levels. A similar condition exists during the landing cycle if the aircraft maintains a holding pattern, since there again the power level must be abnormally low (on a percentage thrust basis) due to the high installed thrust level.
The problem is further compounded, however, since not only does the low combustor inlet temperature result in increased exhaust emissions, but it also degrades the aircraft anti-icing system effectiveness. Some aircraft and engine surfaces are normally heated by air bled from the combustor inlet and if this air is too cool the heating process does not function properly.